This invention defines a particular novel arrangement, which can reduce drilling costs in deep ocean and greatly improve the safe handling of the hydrocarbon gas or liquids that may escape the subsurface formation below seabed and then pumped from the subsurface formation with the drilling fluid to the drilling installation that floats on the ocean surface. By performing drilling operations with this novel arrangement as claimed, there is provided a complete new way of controlling the pressure in the bottom of the well and at the same time safely and efficiently handling hydrocarbons in the drilling riser system. The arrangement comprises the use of prior known art but arranged so that totally new drilling methods is achieved. By arranging the various systems coupled to the drilling riser in this particular way, totally new and never before used methods can be performed safely in deepwater. The invention relates to a deep water drilling system, and more specifically to an arrangement for use in drilling of oil/gas wells, especially for deep water wells, preferably deeper than 500 m water-depth.
Experience from deepwater drilling operations has shown that the subsurface formations to be drilled usually have a fracture strength close to that of the pressure caused by a column of seawater.
As the hole deepens the difference between the formation pore pressure and the formation fracture pressure remains low. The low margin dictates that frequent and multiple casing strings have to be set in order to isolate the upper rock sections that have lower strength from the hydraulic pressure exerted by the drilling fluid that is used to control the larger formation pressures deeper in the well. In addition to the 50 static hydraulic pressure acting on the formation from a standing column of fluid in the well bore there are also the dynamic pressures created when circulating fluid through the drill bit. These dynamic pressures acting on the bottom of the hole are created when drill fluid is pumped through the drill bit and up the annulus between the drill string and formation. The magnitude of these forces depends on several factors such as the rheology of the fluid, the velocity of the fluid being pumped up the annulus, drilling speed and the characteristics of the well bore/hole. Particularly for smaller diameter hole sizes these additional dynamic forces become significant. Presently these forces are controlled by drilling relatively large holes thereby keeping the annular velocity of the drilling fluid low and by adjusting the rheology of the drilling fluid. The formula for calculating these dynamic pressures is stated in the following detailed description. This new pressure seen by the formation in the bottom of the hole caused by the drilling process is often referred to as Equivalent Circulating Density (ECD).
In all present drilling operations to date in offshore deepwater wells, the bottom of the well will observe the combined hydrostatic pressure exerted by the column of fluid from the drilling vessel to the bottom of the well, plus the additional pressures due to circulation. A drilling riser that connects the seabed wellhead with the drilling vessel contains this drilling fluid. The bottom-hole pressure to overcome the formation pressure is regulated by increasing or decreasing the density of the drilling fluids in conventional drilling until the casing has to be set in order to avoid fracturing the formation.
In order to safely conduct a drilling operation there has to be a minimum of two barriers in the well. The primary barrier will be the drilling fluid in the borehole with sufficient density to control the formation pressure, also necessary in the event that the drilling riser is disconnected from the wellhead. This difference in pressure caused by the difference in density between seawater and the drilling fluid can be substantial in deep water. The second barrier will be the blowout preventer BOP (BOP) in case the primary barrier is lost.
As the drilling fluid must have a specific gravity such that the fluid remaining in the well is still heavy enough to control the formation when the drilling marine riser is disconnected, this creates a problem when drilling in deep waters. This is due to the fact that the marine riser will be full of heavy mud when connected to the sub-sea blowout preventer, causing a higher bottom-hole pressure than required for formation control. This results in the need to set frequent casings in the upper part of the hole since the formation cannot support the higher mudweight from the surface.
In order to be able to drill wells with a higher density drilling fluid than necessary, multiple casings will be installed in the borehole for isolation of weak formation zones.
The consequences of multiple casing strings will be that each new casing reduces the borehole diameter. Hence the top section must be large in order to drill the well to its planned depth. This also means that slimhole or slender wells are difficult to construct with present methods in deeper waters.
Normally it is not possible to control the pressure from the surface in a conventional drilling operation, due to the fact that the well returns will flow into an open flow line at atmospheric pressure. In order to obtain wellhead pressure control, the well return has to be routed through a closed flow line by way of a closed blowout preventer to a choke manifold. The advantage of controlling bottom hole pressure by means of wellhead pressure control is that a pressure change at the surface results in an almost instantaneous pressure response at the bottom of the hole when a single phase drilling fluid is used. In general, the surface pressure should be kept as low as possible to obtain safer working environment for the personnel working on the rig. So, it is preferable to control the well by changing pressures in the well bore to the largest extent. Conventionally, this can be performed by means of hydrostatic pressure control and friction pressure control in the annulus.
Hydrostatic pressure control is the prime means of bottom hole pressure control in conventional drilling. The mud weight will be adjusted so that the well is in an overbalanced condition in the well when no drilling fluid circulation takes place. If needed, the mud weight/density can be changed depending on formation pressures. However, this is a time consuming process and requires adding chemicals and weighting materials to the drilling mud.
The other method for bottom hole pressure control is friction pressure control. Higher circulating rates generate higher friction pressure and consequently higher pressures in the bore hole. A change in pump rate will result in a rapid change in the bottom hole pressure (BHP). The disadvantage of using frictional pressure control is that control is lost when drilling fluid circulation is stopped. Frictional pressure loss is also limited by the maximum pump rate, the pressure rating of the pump and by the maximum flow through the down hole assembly.
The only reference referring to neutralization of ECD effects is found in SPE paper LIDC/SPE 47821. Reference in this paper is made to WO 99/18327.
All and each of the above references are hereby incorporated by reference.